1. Technical Field
The present invention relates to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators, and to methods of manufacturing the stators.
2. Description of Related Art
Conventionally, there are known stators for electric rotating machines which include a hollow cylindrical stator core, a stator coil, and an outer cylinder.
The stator core is comprised of a plurality of stator core segments that are arranged in the circumferential direction of the stator core to adjoin one another in the circumferential direction. Further, to reduce iron loss of the stator core, each of the stator core segments is formed by laminating a plurality of magnetic steel sheets in the axial direction of the stator core. Moreover, the stator core has a plurality of slots that are formed in the radially inner surface of the stator core so as to be spaced from one another in the circumferential direction of the stator core. The stator coil is mounted on the stator core so as to be received in the slots of the stator core. The outer cylinder is fitted on the radially outer surfaces of the stator core segments so as to fasten them together.
Moreover, there is also known, for example from Japanese Patent Application Publication No. 2009-225504, a method of shrink-fitting the outer cylinder on the radially outer surfaces of the stator core segments. More specifically, according to the method, the inner diameter of the outer cylinder is set to be less than the outer diameter of the stator core. In the shrink-fitting process, the outer cylinder is first heated, thereby causing the inner diameter of the outer cylinder to become greater than the outer diameter of the stator core. Then, the outer cylinder is fitted onto the radially outer surfaces of the stator core segments all of which together make up the radially outer surface of the stator core. Thereafter, the outer cylinder is cooled at room temperature until the difference in temperature between the outer cylinder and the stator core segments becomes zero. As a result, the stator core segments are fixed together by means of compressive stress induced by the difference between the inner diameter of the outer cylinder and the outer diameter of the stator core.
However, with the above method, buckling of the magnetic steel sheets that are laminated to form the stator core segments may occur when there are variations in the dimensions of the magnetic steel sheets and the outer cylinder.
Specifically, referring to FIG. 12, the fastening force of the outer cylinder 37A is applied radially inward to the magnetic steel sheets 36A, inducing compressive stress in the magnetic steel sheets 36A in the circumferential direction of the stator core. When there are variations in the dimensions (e.g., diameters) of the magnetic steel sheets 36A and the outer cylinder 37A, the circumferential compressive stress induced in the magnetic steel sheets 36A may become excessively large, causing the magnetic steel sheets 36A to be buckled (or deformed) in the thickness direction thereof (i.e., in the axial direction of the stator core).
Moreover, for each of the stator core segments 32A, the magnetic steel sheets 36A which are laminated to form the stator core segment 32A may be fixed together by, for example, staking or welding. However, when buckling of the magnetic steel sheets 36A occurs, the force of fixing the magnetic steel sheets 36A together will be weakened. Consequently, in the worst cases, the buckled magnetic steel sheets 32A will be separated from the other magnetic steel sheets 32A.
To solve the above problem, one may consider increasing the number of staking spots or welding spots in the magnetic steel sheets 36A. However, with the increase in the number of the staking spots or welding spots, iron loss (or eddy-current loss) of the stator core would be increased.